Let us think about the role of soma in this fascinating novel. We are shown that soma is consumed by the citizens whenever they are feeling slightly unahppy or are questioning what is going on around them. Soma dispenses with such negative emotions and doubts by filling humans with happiness and restoring them to a blissful state where such doubts vanish, allowing them to enjoy their lives with a child-like playful innocence. Consider, for example, the way that soma is described in Chapter 17 in the conversation between Mustapha and Mond:

And if ever, by some unlucky chance, anything unpleasant should somehow happen, why, there’s always soma to give you a holiday from the facts. And there’s always soma to calm your anger, to reconcile you to your enemies, to make you patient and long-suffering. In the past you could only accomplish these things by making a great effort and after years of hard moral training. Now, you swallow two or three half-gramme tablets, and there you are. Anybody can be virtuous now. You can carry at least half your morality about in a bottle. Christianity without tears—that’s what soma is.

It is interesting that Mustapha compares soma to Christianity, but "without tears." Actually, if we look carefully at the way he talks about soma, one way of viewing it could be as a way of maintaining the status quo. Soma gives citizens "a holiday from the facts." It doesn't change society but it does temporarily blind its citizens to the facts of society that they find so hard to accept. A really interesting thesis statement would be something like this:

Soma in Brave New World is shown to be a method of preserving the status quo and controlling its citizens so they don't desire to change society.

This would allow you to analyse soma's role and function in this new society and how it is used by the government to suppress any revolutionary ideas that its citizens may have.